The present invention relates generally to electrical transmission system protection, and more particularly to a phasor measurement units (PMU)-based controlled system separation method to protect against a catastrophic blackout.
A major goal in transmission system protection is to avoid cascading failures. Large power system blackouts, although infrequent, may influence up to tens of millions of people and result in direct costs up to billions of dollars. There are also indirect costs such as possible social disruptions and the propagation of failures into other infrastructures such as communication, water supply, natural gas, and transportation systems. The vital importance of electric power to our society motivates continued attention to maintaining power system stability and developing effective self-healing protection and control strategies to prevent blackouts under major disturbances (either nature or man-made).
Historically, major blackout events started with cascading failures of transmission components in a power system. Those failures were often dependent. The first few failures were mainly caused by unanticipated disturbance events (e.g. short-circuit faults due to tree-to-line contacts and transmission outages due to severe weather conditions); thus, some transmission components became out-of-service thereafter, and as a result, overloads occurred on some other transmission components causing overload protection to trip those components and generate more overloads and failures such that the failures cascaded and spread to a wide area. The cascading failures gradually weakened the connection of the power system and caused unintentional system separation and severe stability problems leading to a large-area blackout.
In recent years, efforts have been made to mitigate cascading failures in order to prevent blackouts. When cascading failures occur, it is difficult for system operators at control centers to take corrective actions in a matter of a few minutes or even seconds; thus, automatic protection and control schemes are required in preventing, slowing, or mitigating cascading failures. These automatic schemes need to be able to strategically coordinate local protection actions at the system level in order to stop the spread of cascading failures effectively. However, protecting interconnected power systems against cascading failures and the stability problems incurred poses a challenge to power system engineers because power systems in the real world are often huge and complex while the time for online computations and decision making for control actions is quite limited.
One of the most severe stability problems that may occur with the spread of cascading failures is loss of synchronism. When cascading failures continuously weaken the power system and impact the connections between interconnected sub-systems or control areas, inter-area oscillations will happen and grow. If not damped, oscillations will evolve into angle separation between two or more groups of generators, namely loss of synchronism, to result in outages of generators and transmission devices. In an unpredictable way, the system may collapse and separate into electrical islands. This is called unintentional system separation.
Because the formation of those islands is not in a designed manner, it is unavoidable to cause problems such as having (1) large imbalances between generation and load exist in some islands (Excessive load in a load-rich island has to be shed timely to prevent rapid frequency declines of generators while excessive generation has to be rejected in a generation-rich island. Consequently, large-area power outages may be caused by the unplanned island formation.); (2) some transmission lines being overloaded and then tripped—possibly resulting in more failures and further system separation within islands; and (3) the possibility of the generators that tend to lose synchronism in one island formed, making it difficult for them to cohere with each other spontaneously—which could lead to further outages of generators or transmission devices, and even worse, separation may continue in that island.
The above shows that unintentional system separation may not stop cascading failures. In contrast, it may worsen the situation and lead to large-area blackouts. For example, during the European blackout event on Nov. 4, 2006, one of the most severe and largest disturbances in Europe, the UCTE (Union for the Co-ordination of Transmission of Electricity) transmission grid spontaneously separated into three islands (West, North-East and South-East). The separation was caused by cascading line trips throughout the UCTE area. All the line trips were performed by local distance protections due to overloads. After the separation, significant power imbalances existed in islands: the Western and South-East islands are rich in load and the North-East island is rich in generation. The large power imbalance in the Western island induced a severe frequency drop and caused an interruption of supply for more than 15 million European households.